1. Field of the Invention
This invention pertains to ammunition, explosive devices, and reaction motor power plants having a casing opened by a safety device when the casing is externally heated so that propellant or explosive within the casing is vented so as to burn rather than deflagrate or detonate.
2. Description of the Prior Art
Ordnance items present an extreme hazard in the event of "cook-off" which may be defined as the detonation or deflagration of an active material, such as an explosive or a propellant charge, due to external heating in an accidental fire or the like. This hazard is, typically, minimized by providing a casing, which contains the charge, with an arrangement for opening the casing to vent pressure therein before the casing attains a temperature where cook-off may occur. Such a casing contributing to cook-off may be a wall, as of a rocket motor or penetrating warhead, directly in contact with the active material, but may include structure surrounding the motor or warhead.
Prior art cook-off prevention arrangements include rocket motor casings of reinforced plastic which soften and fail on fast, direct cook-off from exposure to flame before a contained propellant attains cook-off temperature. However, this arrangement is ineffective when the casing is subjected to slow cook-off from indirect heating. Another arrangement utilizes a casing with stress riser grooves which cause the casing to open thereat when the casing is subjected to pressure by an explosive therein initially decomposing from heat. This arrangement is effective with relatively weak casings on both fast, direct cook-off and slow, indirect cook-off. However, with a relatively strong casing for target penetration the stress risers cannot weaken the case sufficiently so that venting either does not occur to prevent cook-off or occurs at such a high pressure that nearby structures are damaged and personnel are injured.
Prior art cook-off prevention arrangements also include a casing provided with an orifice having a closure opened or released by melting or thermal stress. Other prior-art arrangements include a casing vented by thermal stress when one side is heated. This stress may be increase by stiffening and thermally insulating portions of the casing. These arrangements, like the stress riser arrangement, are deficient with strong casings. Further prior art arrangements include explosive or other casing penetrators activated at a temperature approaching cook-off. These latter arrangements are effective but may themselves be a hazard and require initiation devices which are relatively complex and may be adversely affected by long storage.